quantities of sediment. Degradation in the system had reached as much as 5 to 6 m (15 to 20
ft) before measures were taken to stabilize the creek.
Stabilization was achieved by flattening the gradient by constructing numerous drop structures
and by reforming the banks with riprap. The system has stabilized but it is a different system.
The channel is straight, much of the vegetation has been washed away, and the natural
sequence of riffles and pools has been destroyed. The valley may never again have the
natural form and beauty it once possessed. We should bear in mind that diversions to or from
the natural river system can greatly alter its geometry, beauty and utility. The river may
undergo a complete change, giving rise to a multitude of problems in connection with the
design and maintenance of hydraulic structures, encroachments and bridge crossings along
the affected reach.
In the preceding paragraphs, possible immediate and long-term responses of river systems to
various types of river development have been described, but no guidance has been given on
how to determine the magnitude of these changes. This important aspect of the response of
rivers to development is treated in detail in later chapters.
1.5 TECHNICAL ASPECTS
Effects of river development, flood control measures and channel structures built during the
last century have proven the need for considering delayed and far-reaching effects of any
alteration humans make in a natural alluvial river system.
Because of the complexity of the processes occurring with natural channel flows and the
accompanying erosion and deposition of material, an analytical approach to the problem can
be very difficult and time consuming. Most of our river process relations have been derived
empirically. Nevertheless, if a greater understanding of the principles governing the processes
of river formation is to be gained, the empirically derived relations must be put in the proper
context by employing an analytical approach.
Attempts at controlling large rivers have often led to the situation described by J. Hoover
Mackin (1937) when he wrote:
"The engineer who alters natural equilibrium relations by diversion or damming
or channel improvement measures will often find that he has the bull by the tail
and is unable to let go. . . . . . as he continues to correct or suppress
undesirable phases of the chain reaction of the stream to the initial 'stress' he
will necessarily place increasing emphasis on study of the genetic aspects of
the equilibrium in order that he may work with rivers, rather than merely on
them."
Through such experiences, one realizes that, to prevent or reduce the detrimental effects of
any modification of the natural processes and state of equilibrium on a river, one must gain an
understanding of the physical laws governing them and become knowledgeable of the
far-reaching effects of any attempt to control or modify a river's course.
1.13
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